soc: atmel_sam0: samd21: Fix interrupt line count

The Atmel SAM D21 SoC, according to the original Atmel datasheet
(Atmel-42181N), has 28 interrupt lines (0-27).

There have been mysterious changes in the number of interrupt lines and
on-chip peripherals in the recent Microchip datasheet releases, but
there is no explicit information available for this (e.g. PCN), so we
take the safest approach by assuming the lowest interrupt line number.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>